Considerable research, development and production efforts have been dedicated to the manufacture of polymeric materials. Often these efforts are directed to developing polymers suited for particular purposes and having defined characteristics for achieving specific results. Selection of starting materials, compounding techniques, fabrication methods and post-fabrication activities lead to tailoring the materials for an end product. For example, differing blending techniques of identical starting material can significantly alter the end product's physical characteristics. Employing cross-linking methods during the course of manufacture also tends to present dramatic variations in the material'properties.
Cross-linking is a shorthand term to describe the formation of a three-dimensional molecular network within a polymeric composition. Since Charles Goodyear first cross-linked rubber with sulfur, many techniques and materials have been developed and now contribute to an important technology within the field of polymer chemistry. Conventional techniques employed to effectuate cross-linking generally involve incorporation of thermally-activated, multi-functional chemical additives within a cross-linkable material and subsequently heating the material above the temperature of activation to form covalent bonds within the polymeric network. Alternatively, the polymeric material may be cured when exposed to ionizing radiation of appropriate energy.
The presence of these covalent bonds generally enhance various desirable physical characteristics of a polymeric material such as increased resistance to thermal and solvent degradation, structural strength, shelf-life retention, etc. Additionally, curing may impart new characteristics such as heat-recoverability. Thus, cross-linking permits a material to be tailored to possess specific characteristics for particular uses. Enhancement of such characteristics, however, often generates other problems. For example, the additional structural strength is generally equated to increased hardness. Therefore, cross-linked materials generally do not possess a low degree of hardness.
Difficulties often arise during fabrication of cross-linked materials by conventional methods. The additional material rigidity and strength detracts from subsequent conventional fabrication techniques such as rolling and extrusion. For example, due to increased frictional forces, an article formed by extrusion of a cross-linked polymeric material often exhibits melt fracture (crack lines along the outer surfaces). The melt fracture is induced by considerable frictional forces resulting from forcing the material, under pressure and at an elevated temperature, through a mandrel or die. These frictional forces may contain sufficient energy to overcome the energy of the covalent bonds and may result in surface roughness of the fabricated article.
On the positive side, cross-linking contributes markedly to a very desirable property shelf-life retention. Shelf-life retention of a fabricated article is that property identified as the ability of the article to preserve its shape during storage. Uncross-linked materials, particularly elastomers, generally have an extremely limited retention capacity, if any at all. For this reason, fabricated elastomeric articles must be stored in a carefully controlled environment or be used immediately so that the original article configuration remains substantially unchanged. Those articles produced from elastomeric or rubbery materials which possess some degree of shelf-life retention, often involve extended processing periods (particularly molding) to achieve measurable structural integrity and shelf-life retention. Cross-linking an elastomer partially overcomes this problem. As noted above, however, the material becomes correspondingly harder as the degree of cross-linking increases.
For example, molding periods for such materials may exceed five to ten times the period necessary for molding ordinary thermoplastics.